Dynamic link assignment in a communication system

ABSTRACT

An architecture for the dynamic assignment of links in a multi-user communication system. A plurality of information channels are provided in a forward communication link of the communication system for carrying channel information of the plurality of information channels from a transmitter to a plurality of corresponding receiving devices. The channel information in corresponding select ones of the plurality of information channels is varied dynamically in response to link conditions of the associated receiving devices to more efficiently utilize the channel bandwidth.

This application claims priority under 35 U.S.C. § 119(e) from U.S.Provisional Patent application Ser. No. 60/220,261 entitled “DynamicLink Assignment” and filed Jul. 24, 2000.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention is related to digital communication systems, and moreparticularly to multi-user satellite systems for providing user accessto a global packet-switched data network.

2. Background of the Art

The advent of the Internet and the commercial opportunities offered byreaching the millions of potentially new customers which connect theretohave motivated some companies to provide wireless connectivity for thoseusers which cannot use conventional means hardwired means. For example,satellite-based systems provide a mechanism whereby users who are onlyoffered conventional relatively low modem speed access or have noalternative for connecting at all, can now connect to such packet-basedsystems at higher speeds.

However, inefficient use of resources in multi-user satellite systemsresults in excessive link margins that drastically reduce systemcapacity. Typically, the forward link from the satellite to the user isa time-multiplexed data stream that is received by a large number ofuser terminals. As such, the satellite must be capable of providingservice to the user that is under the lowest quality link conditions.Existing satellite communication packet-based systems which offer accessto the Internet can transmit digital information to users in unicast,that is, the digital information can be sent to a specific user basedupon a unique identification number (ID) assigned to that user, theunique user ID derived via any number of conventional methods. However,existing unicast transmissions still fail to efficiently utilize theavailable bandwidth by formatting and sending the unicast data underconstraints, which anticipate the worst possible reception conditionsfor any user to reasonably ensure that all users can receive thetransmission. This “one-size-fits-all” problem requires satellitesystems to operate with link margin requirements that are extremelywasteful to system capacity.

What is needed is a link architecture that allows the link to becustomized on a per-user basis to more efficiently utilize channelbandwidth in the communication system.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein, in one aspectthereof, comprises architecture for the dynamic assignment of links in amulti-user communication system. A plurality of information channels areprovided in a forward communication link of the communication system forcarrying channel information of the plurality of information channelsfrom a transmitter to a plurality of corresponding receiving devices.The channel information in corresponding select ones of the plurality ofinformation channels is varied dynamically in response to linkconditions of the associated receiving devices to more efficientlyutilize the channel bandwidth.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a block diagram of a frequency channelization scheme,in accordance with a disclosed embodiment;

FIG. 2 illustrates a flow chart of the process for dynamicallycontrolling a user link in accordance with present link conditions;

FIG. 3 illustrates a graph of an OFDM waveform and channel numberingscheme based around a center frequency;

FIG. 4 illustrates a graph of frequency response of a simulated dynamiclink assignment waveform;

FIG. 5 illustrates organization of the various slots utilized in aframe;

FIG. 6 illustrates a diagram of a Synchronization slot;

FIG. 7 illustrates a frame structure of a Receiver Access Channel slot;

FIG. 8 illustrates a diagram of a Frame Definition slot;

FIG. 9 illustrates a diagram of a receiver User/Message Definition slot;and

FIG. 10 illustrates an example of the channel/slot structure of thedynamic link assignment architecture.

DETAILED DESCRIPTION OF THE INVENTION

The disclosed Dynamic Link Assignment (DLA) architecture provides thecapability of more than quadrupling channel capacity in a multi-channelsystem by presenting a multi-user access scheme that allows thecommunication system to dynamically customize, without requiringresynchronization and associated loss of data, a user waveform to matchthe user link conditions.

In a satellite-based application, the architecture allows variablemodulation and coding formats on a per-user basis through the use ofTime Division Multiplexing (TDM) and Orthogonal Frequency DivisionMultiplexing (OFDM). A user terminal provides feedback to the satellitesystem such that the forward link to the user can be customizeddynamically according to link conditions at any particular moment.Moreover, as the OFDM waveform is frequency and time locked, a user canchange modulation and coding rapidly without resynchronization. Carrierand timing synchronization is achieved on a central, data-bearingchannel. This arrangement allows the overall forward link to becustomized on a per-user basis, allowing for reduced operating margin.Additionally, a combination of modulation and turbo coding providesbandwidth and power efficiency that approach Shannon's limit. Althoughthe following discussion focuses on satellite-based systems, thedisclosed architecture is not restricted to satellite systems, but hasapplication in any multi-user digital communication system in which datatransmission is to a number of users each operating under differentconditions, e.g., a passive optical network.

Referring now to FIG. 1, there is illustrated a general block diagram ofa channelization scheme 100, in accordance with a disclosed embodiment.In OFDM, a subcarrier pulse 102 used for transmission of information ischosen to be rectangular, which shape has the advantage that pulseforming and modulation at the head-end transmitter can be performed byan Inverse Discrete Fourier Transform (IDFT) that can be implementedvery efficiently as an Inverse Fast Fourier Transform (IFFT).Accordingly, at the receiver, an FFT is needed to reverse (ordemultiplex) the channels. Leading and trailing guard bands 104 are usedto combat multipath signals.

In general, the overall bandwidth per primary channel 106 isapproximately x MHz, and each primary channel 106 is subdivided into nsub-channels S (denoted 108, and where n=0, . . . , p), that overlap inan OFDM sense, resulting in a symbol rate of x/n M-symbols/sec (Msps)sub-channel. Within each sub-channel S_(n) 108, a frame structure isdefined (and is discussed in greater detail hereinbelow) such that thereare 2^(z) symbols per frame, where z is selected for optimal signalquality. The channel numbering scheme is based around a center frequencyfc (denoted 110), such that a first sub-channel S₀ 112 is centered atthe center frequency 110. The remaining sub-channels 108 are distributedabout the center frequency 110 as illustrated in FIG. 1.

Referring now to FIG. 2, there is illustrated a flow chart of theprocess for dynamically controlling a user link in accordance withpresent link conditions. Flow begins at a starting point 200 andcontinues to a function block 202 where the user terminal (orground-based terminal wherever it may be located) determines the currentchannel transmission parameters based upon the existing link conditionsfor that user location. Flow is to a decision block 204 to thendetermine if link conditions for that user channel have changed. If not,flow is out the “N” path to a function block 206 to maintain the currentchannel parameters for that user. Flow is then back to the input offunction block 202 where the user terminal again determines the linkconditions. On the other hand, if the link conditions for that channelhave changed, flow is out the “Y” path of decision block 204 to afunction block 208 where the satellite hub receives the current linkparameters for that channel as a link status signal. The hub thenadjusts the signal channel for optimum operating parameters according tocurrent link conditions, as indicated in a function block 210. Flow isthen to a function block 212 to transmit the user channel information tothe user terminal under the adjusted channel parameters.

The channel adjustment process is performed dynamically in response toexisting link conditions for that particular user terminal. It can beappreciated that in a power-up scenario, or where the link between theuser terminal and satellite hub is lost, a synchronization processoccurs under default operating condition to establish the link as soonas possible. To that end, a feedback path exists between the userterminal and satellite hub wherein the link conditions for thatparticular user are being continually monitored such that the forwardlink for that user channel can be adjusted to ensure optimum channelquality under existing link conditions. The return path from theground-based user terminal to the satellite can be a direct wirelesspath from the user transmitter (e.g., a satellite dish system) to thesatellite hub. Alternatively, the link from the user terminal can be viaother conventional means such as a return path through a telephone lineto an access provider who then completes the return link to thesatellite hub. Other methods for providing the return path from the userto the satellite are commonly known by those skilled in the art.

Waveform Description

Referring now to FIG. 3, there is illustrated a graph of an OFDMwaveform 300 and channel numbering scheme based around the centerfrequency fc 110, in accordance with a disclosed embodiment. Note thatchannel zero 112 is defined as the channel that is centered on thecenter frequency 110. The bandwidth 302 of the main lobe 106 isnominally 54 MHz with a null-to-null bandwidth 304 of 57.375 MHz. In theunfiltered case, the first side lobes 306 are approximately 18 dB down(i.e., −18 dB) from the main lobe. In order to maintain accuratesynchronization, the DLA waveform is constrained to require a specialwaveform in the central channel. The central channel 112 is received atthe baseband, and uses a special waveform in order to maintainsynchronization. The waveform in channel zero 112 consists of QPSK(Quadrature Phase Shift Key) data (no constraint on coding or gain),with some side information to aide in synchronization. Information mustbe present in all channel-zero 112 slots. In cases where the channels108 do not fit “evenly” into the primary band 106, a partial channel(not shown) is supported. For partial DLA channels, channel zero 112must be present.

Referring now to FIG. 4, there is illustrated a graph 400 of thefrequency response of a simulated DLA waveform in the example of FIG. 3.The main lobe 106 has a bandwidth of approximately 54 MHz with the firstside lobes 306 down approximately 18 dB from the main lobe 106.

Framing Description

Referring now to FIG. 5, there is illustrated the channelization framestructure. The DLA architecture provides a number of slot and packettypes for use within the frame 500 to allow users to enter and exit thetransmission system, and to provide the customized user link. The DLAslot types include one or more of the following: a Synchronization slot,a Receive Access Channel (RAC) slot, a Frame Definition State (FDS)slot, and DLA User/Message (U/M) slot.

The Synchronization slot appears as the first slot 502 once per frame500 to allow reliable modem synchronization. The RAC slot is in thesecond slot 504, and is a reliable slot that contains user ID tables toallow users to enter the transmission system for data reception in thecurrent frame 500. The entry information for both single-user IDs andbroadcast/multicast IDs are supported in the RAC slot 504. In additionto system entry, the RAC slot 504 provides for a low-latencyhardware-messaging path. Two FDS slots 506 and 508 contain informationregarding the location (in time and frequency) of slots in the nextframe, and the format (modulation, coding, and gain) of user slots inthe current frame 500. The FDS slots 506 and 508 appear as the third andfourth slots on each frequency sub-channel 108. A number of U/M slots510 (U/M1, . . . , U/Mn) contain the user transport stream payload, andcomprise two classes of user slots: a single user per slot and amulti-user slot to handle low data rate traffic such as voice. Thesingle user slot may be directed toward an individual terminal, or maybe a broadcast or multicast slot as originally defined by the RAC slot504.

DLA Synchronization Slot

Referring now to FIG. 6, there is illustrated a diagram of asynchronization frame 600. The synchronization frame 600 appears as thefirst slot 502 of each channel 108 and each frame 500, and consists of apreamble filed 602 that contains BPSK (Binary Phase Shift Key) ones for3,841 symbols, the utilization of which allows the terminal demodulatorto acquire the carrier frequency and phase, as well as the symboltiming.

Following the preamble field 602 is a Unique Word (UW) field 604 thatsignifies the beginning of the frame 500. The UW field 604 consists of255 BPSK symbols, and is generated via an 8-bit linear feedback shiftregister with a polynomial value of x⁸+x⁴+x³+x²+1, and a seed value of0x10. The UW frame 600 is sufficient for reliable frame detection at achannel Signal-to-Noise Ratio (SNR) that corresponds to the most powerefficient modulation and coding, specifically an SNR=−3.0 dB. A harddecision parallel correlator with a programmable threshold is thepreferred approach for acquiring frame synchronization.

DLA Receiver Access Channel Slot

Referring now to FIG. 7, there is illustrated a structure of a ReceiverAccess Channel frame 700. The RAC frame 700 contains information thatallows users to enter the transmission system or receive messages basedupon the user ID or broadcast ID. The following constraints are placedon the RAC frame 700: (1) data in RAC frame 700 that is repeated acrosssub-channels 108 is rotated from sub-channel to sub-channel to prevent apower surge in the DLA link, and (2) broadcast and multicast IDinformation must only occur in the sub-channel 108 that is equivalent tothe upper four bits of the broadcast/multicast ID. This provides forease of use and entry into broadcast/multicast data streams.

Starting in slot two 504 of every frame 500, each RAC frame 700 containsa set of individual user IDs and a smaller set of broadcast IDs. The RACframe 700 contains 4,096 QPSK symbols encoded with two code blocks 702and 704 of (4096, 1331) TPC (Turbo Product Code) data, each having a setof 1,331 information bits (706 and 708), totaling 2,662 informationbits, and each having 2,765 corresponding code bits (710 and 712). Thisallows for forty user IDs (Users 0–39) in each RAC frame 700, or 128 newusers per second.

Each set of information bits 706 (and 708) contains a 16-bit RAC Header714 which is the first sixteen bits of each TPC block 702 and 704. Thefirst eight bits of the header 714 indicate a frame counter 715, and thenext eight spare bits 717 of the header 714 are reserved for future use.There are twenty User fields 716 (User 0–19) per set of information bits706 (and 708), and each User field 716 contains sixty-four bits: a48-bit User ID 718, an 8-bit Control field 720, and an 8-bit Data field722. Each of the User fields 716 contains information for an individualuser, multicast users, or broadcast users. The 48-bit User ID (orBroadcast ID) field 718 conforms to the IEEE 802.3 standard. Each user,broadcast, and multicast is uniquely identified by the User ID 718 orphysical MAC (Media Access Control) address. The broadcast and multicastIDs are made available to registered users and stored in a data file onthe terminal computer. The four most-significant bits of the broadcastand multicast IDs correspond to the channel on which the broadcast istransmitted. The Control byte field 720 is a control command, and isdiscussed in greater detail with respect to messaging. The primarypurpose of the Data byte 722 is to identify the slot number in which theuser data or message appears in the current frame. However, for certaincontrol commands, the Data field 722 can contain other data, which isdiscussed in greater detail hereinbelow with respect to messaging. Thereare two 32-bit CRC (Cyclic Redundancy Check) fields 724, one for eachset of information bits 706 and 708 which provide error detection forthe header 714 and the twenty user information packets 716, in theirrespective TPC blocks 702 and 704. There are also two 3-bit zero padfields 719, one for each set of information bits 706 and 70-8 whichserve to fill out the TPC blocks.

DLA Frame Definition State Slot

Referring now to FIG. 8, there is illustrated a diagram of the third andfourth slots 506 and 508, the Frame Definition State slots. The thirdand fourth slots (506 and 508, respectively) in each channel are the FDSslots, and each contains the modulation, coding, gain, next channel, andnext slot information for each user within a channel. Each FDS slot 506and 508 contains 4,096 QPSK symbols, and each set of 4,096 QPSK symbolscorresponds to two blocks of (4096, 1331) TPC coded data. A first TPCblock 800 of the first FDS frame 506 contains 1,331 information bits 810and 2,765 corresponding code bits 812. A second TPC block 804 of thefirst FDS frame 506 contains 1,331 information bits 814 and 2,765corresponding code bits 816. A first TPC block 806 of the second FDSframe 508 contains 1,331 information bits 818 and 2,765 correspondingcode bits 820. A second TPC block 808 of the second FDS frame 508contains 1,331 information bits 822 and 2,765 corresponding code bits824. This provides 2,662 information bits for each of the two FDS slots506 and 508, for a total of 5,324 information bits.

Each set of information bits (810, 814, 818 and 822) further subdividesinto sixty-four 20-bit Slot Definition fields which contain informationabout user slots [4 . . . 255], a Spare bits field 828 of sixteen sparebits, a 32-bit CRC field 830 for error detection over the previoussixteen spare bit fields 828, sixty-four slot definition fields 826, anda 3-bit zero pad field 831. The CRC field 724 adds an additional layerof error checking to prevent spurious jumps from frame to frame.Information for slots [0 . . . 3] provide default settings. Each 20-bitSlot Definition field 826 is divided into the following three subfields:an 8-bit Modulation, Coding, and Gain field 832 which specifies themodulation, TPC coding, and gain format of the user slot in the currentframe (the default value in slots [0 . . . 3] is 0x01) (the 8-bit valueis extracted by the terminal and decoded to three distinct configurationvalues that are used by the terminal to set-up the user slots); a 4-bitNext Channel field 834 that indicates which channel the user slot willuse in the next frame (the default value in slots [0 . . . 3] is 0x00);and an 8-bit Next Slot field 836 that indicates which time slot the userslot will use in the next frame. If the Next Channel field 834 and NextSlot field 836 point to the primary RAC channel, the user goes to theRAC in the next frame.

DLA Receiver User/Message (U/M) Slots

Referring now to FIG. 9, there is illustrated a diagram of a userdefinition slot. The transport stream appears at the user slot level andis based upon a custom transport stream structure. The DLA transportstream structure varies based on the combination of modulation and TPCcoding used on the channel. Each U/M slot 900 contains one or more TPCblocks 902. FIG. 9 illustrates a U/M slot 900 containing four TPC blocks902. Each TPC block 902 contains a standard Header 904, Payload data906, a CRC 908, and Parity bits 910. The 32-bit Header field 904contains header information for the user slot, which user slotinformation is described using three sub-fields: an 11-bitStart-of-Protocol Packet pointer 912 which is used to point to a bytelocation in the payload filed which is the first byte of a higher layerprotocol packet (IP, for example), and if no start-of-packet occurs inthe TPC block, this protocol pointer field 912 is set to 0×7FF; a 13-bitLength Field 914 which identifies the length (in bytes) of theinformation payload 906 (and is used by the device driver to determinewhat data to pass to the higher layers in the protocol stack); and an8-bit Next Slot Number filed 916 which identifies the next valid U/Mslot for the user in the current frame. If it is the last slot for theuser in the particular frame, this value is set to 0x00.

The size of the Payload field 906 ranges from 644 to 15,208 bits. Thisvariable-length field 906 contains the payload that is used fortransporting data or messages. Software ensures that the length of validdata in the payload field 906 is always an integral number of bytes. The32-bit CRC field 908 provides error detection for the header 904 andpayload 906 of the slot 900. The Parity field 910 is a variable-lengthfield, which contains the TPC parity bits.

Referring now to FIG. 10, there is illustrated an example of thechannel/slot structure of the dynamic link assignment architecture. Each54 MHz primary band 106 contains sixteen frequency-multiplexedsub-channels 108 that partially overlap in an OFDM fashion, providingbandwidth efficiency near Nyquist requirements. In this particularembodiment, the sixteen sub-channels 108 are modulated in the main lobe106 of the subcarrier pulse 102. Each sub-channel 108 operates at1/16^(th) of the 54 MHz channel frequency providing a symbol rate of54/16=3.375 Msps. The nominal capacity C_(nom) of the x=54 MHz primarychannel is calculated assuming a nominal modulation and coding thatyields 2.5 bits/symbol. The nominal capacity C_(nom) is calculated asfollows:C_(nom)=(2.5 bits/symbol)(3.375 Msps/channel)(16 channels/compositesignal)=135 Mbps.Each sub-channel frame 1000 is structured to facilitate the disclosedlink architecture. For example, in a channel 1002, the correspondingframe 1004 (for a single frame period of 0.311 seconds) begins with asynchronization frame 1006, followed by a RAC slot 1008, two FDS slots1010 and 1012, and multiple user slots 1014. All other channels have thesimilar frame structure.

Note that the disclosed architecture can be implemented in hardware suchthat one or more digital devices are fabricated to provide a high speedsolution (e.g., digital CMOS chip).

The disclosed architecture, in general, has application in any point tomulti-point digital communications link in which the “multi-points” havedifferent link conditions and feedback is provided to monitor andcontrol the link in response to changing link conditions. For example,an application includes a cellular telephone that uses a point (basestation) to multi-point (cell phones) configuration under various linkconditions (e.g., antenna size, receiver sensitivity, interference,distance to base station, etc.).

The invention also has application where the overall system architectureincludes a multi-point to multi-point configuration, as long as it canbe decomposed into at least one point to multi-point link.

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A method of communicating data in a multi-user communication systemusing orthogonal frequency division multiplexing (OFDM), comprising thesteps of: providing channel information for one or more dynamicallyassigned frequency channels of a plurality of frequency channels in anOFDM communication link; receiving link information identifying linkconditions for one or more devices to receive said channel information;dynamically varying a modulation and coding format associated with saidchannel information in said one or more dynamically assigned frequencychannels in a response to said identified link conditions; andtransmitting said channel information according to said variedmodulation and coding format to said receiving devices in saiddynamically assigned frequency channels using a broadcast communicationsignal; wherein said channel information includes a unique ID, saidunique ID for uniquely identifying each said receiving device; whereinsaid unique ID is utilized to transmit said channel information inunicast to select ones of said plurality of receiving devices or inmulticast to all said receiving devices.
 2. The method of claim 1,wherein said plurality of frequency channels are time divisionmultiplexed.
 3. The method of claim 1, wherein said channel informationin said one or more dynamically assigned frequency channels in thevarying step is varied in gain and frequency in accordance with saididentified link conditions.
 4. The method of claim 1, wherein saidchannel information in said one or more of said plurality of frequencychannels in the varying step is varied in modulation, coding, gain, andfrequency in accordance with said identified link conditions such thatsynchronization of said respective information channel is maintained. 5.The method of claim 1, wherein the communication system is a satellitecommunication systems that transmits said channel information in saidcommunication link that is a wireless forward communication link.
 6. Themethod of claim 1, wherein the varying step is performed on select onesof said plurality of frequency channels utilizing time divisionmultiplexing and orthogonal frequency division multiplexing.
 7. Themethod of claim 1, wherein said frequency channels overlap.
 8. Themethod of claim 1, wherein said channel information comprises data to betransmitted over a frequency channel of said plurality of frequencychannels.
 9. The method of claim 1, further comprising: dynamicallyassigning said one or more frequency channels to said one or morereceiving devices after said modulation and coding format associatedwith said channel information is dynamically varied.
 10. A method ofcommunicating data in a multi-user communication system using orthogonalfrequency division multiplexing (OFDM), comprising the steps of:providing channel information for one or more dynamically assignedfrequency channels in a plurality of frequency channels in an OFDMcommunication link; receiving link information identifying linkconditions for one or more devices to receive said channel information;varying a modulation and coding format associated with said channelinformation in said one or more dynamically assigned frequency channelsin response to said identified link conditions; and transmitting saidchannel information according to said varied modulation and codingformat to said receiving devices in said dynamically assigned frequencychannels using a broadcast communication signal; wherein synchronizationof carrier and timing between transmitter and said receiving devices isachieved utilizing a frequency channel of said plurality of frequencychannels in said communication link.
 11. The method of claim 10, whereinsaid frequency channel includes a plurality of information slots suchthat each said information slot must contain information in order toachieve said synchronization of carrier and timing.
 12. An apparatus forcommunicating data in a multi-user communication system using orthogonalfrequency division multiplexing (OFDM), comprising: a transmitterconfigured to transmit data over one or more dynamically assignedfrequency channels of a plurality of frequency channels in an OFDMcommunication link of the communication system for carrying channelinformation between said transmitter and one or more receiving devices;a receiver for receiving link information identifying link conditionsfor at least one of said one or more receiving devices; and a varyingdevice coupled with said transmitter and said receiver, and configuredto control said transmitter to vary a modulation and coding formatassociated with said channel information in said one or more dynamicallyassigned frequency channels in response to said identified linkconditions; wherein said channel information is transmitted according tosaid varied modulation and coding format with said transmitter to saidone or more receiving devices in said dynamically assigned frequencychannels using a broadcast communication signal; wherein said channelinformation includes a unique ID, said unique ID for uniquelyidentifying each said receiving device; wherein said unique ID isutilized to transmit said channel information in unicast to select onesof said plurality of receiving devices or in multicast to all saidreceiving devices.
 13. The apparatus of claim 12, wherein said pluralityof frequency channels are time division multiplexed.
 14. The apparatusof claim 12, wherein said varying device varies channel information ingain and frequency in accordance with said identified link conditions.15. The apparatus of claim 12, wherein said varying device varieschannel information in modulation, coding, gain, and frequency inaccordance with said identified link conditions such thatsynchronization of said respective information channel is maintained.16. The apparatus of claim 12, wherein the communication system is asatellite communication system that transmits said channel informationin said communication link that is a wireless forward communicationlink.
 17. The apparatus of claim 12, wherein said varying device variesonly modulation and coding in accordance with said identified linkconditions of said corresponding information channel.
 18. The apparatusof claim 12, wherein said varying device varies channel information onselect ones of said plurality of information channels utilizing timedivision multiplexing and orthogonal frequency division multiplexing.19. An apparatus for communicating data in a multi-user communicationsystem using orthogonal frequency division multiplexing (OFDM),comprising: a transmitter configured to transmit data over one or moredynamically assigned frequency channels of a plurality of frequencychannels in an OFDM communication link of the communication system forcarrying channel information between said transmitter and one or morereceiving devices; a receiver for receiving link information identifyinglink conditions for at least one of said one or more receiving devices;and a varying device coupled with said transmitter and said receiver,and configured to control said transmitter to vary a modulation andcoding format associated with said channel information in said one ormore dynamically assigned frequency channels in response to saididentified link conditions; wherein said channel information istransmitted according to said varied modulation and coding format fromsaid transmitter to said one or more receiving devices in saiddynamically assigned frequency channels using a broadcast communicationsignal; and wherein synchronization between said transmitter and saidplurality of receiving devices is achieved utilizing a frequency channelof said plurality of frequency channels in said communication link. 20.The apparatus of claim 19, wherein said frequency channel includes aplurality of information slots such that each said information slot mustcontain information in order to achieve said synchronization, saidsynchronization including both carrier and timing.
 21. A method fordynamically modifying a communication signal in accordance withdifferent link conditions in a communications system, the methodcomprising: associating a unique ID with a data packet to therebyidentify a receiving device for said data packet; receiving informationidentifying a link condition for said receiving device; dynamicallyassigning a modulation and coding format for said data packet to accountfor said identified link condition; and transmitting said data packet,according to said modulation and coding format, in a dynamicallyassigned frequency channel of a broadcast communication signal includinga plurality of frequency channels formatted to carry data to particularusers.
 22. The method of claim 21, wherein said dynamically assignedfrequency channel comprises time slots in said dynamically assignedfrequency channel in an orthogonal frequency division multiplexingcommunication link.
 23. A method for dynamically modifying an orthogonalfrequency division multiplexing waveform including a plurality offrequency channels in a multi-user communications system wherein atleast a subset of devices in said system receive said waveform underdifferent link conditions, the method comprising: associating a uniqueID with information in said waveform to thereby identify a receivingdevice for said information; receiving data identifying link conditionsfor said receiving device; dynamically varying a modulation and codingformat for said information in a dynamically assigned frequency channelof said plurality of frequency channels to adapt said dynamicallyassigned frequency channel to said identified link conditions for saidreceiving device; and transmitting said waveform as a broadcastcommunication signal including said plurality of frequency channels insaid multi-user communications system.
 24. The method of claim 23,further comprising: dynamically assigning said frequency channel to saidreceiving device after said modulation and coding format is dynamicallyvaried.
 25. The method of claim 23, wherein said dynamically assignedfrequency channel comprises time slots in said dynamically assignedfrequency channel.